Electric and hybrid vehicle propulsion systems typically employ an alternating current (AC) driving circuit, such as an inverter, to convert direct current (DC) voltage of an energy storage device to variable speed AC waveforms to drive an electric motor. The driving circuit usually includes power electronic devices such as insulated gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs) as switches to construct AC voltages for driving the electric motor by chopping the DC voltage according to a pulse width modulation (PWM) scheme. The constructed AC voltages normally contain a fundamental wave component (e.g., wave component having a fundamental frequency, usually the lowest frequency, that also corresponds to the desired AC voltage for driving the electric motor) and switching harmonics (e.g., wave components having much higher frequencies than the fundamental frequency) due to the PWM.
Because the AC voltages generated by the driving circuit are constructed from a DC voltage, the amplitude of the fundamental voltage is limited by the DC voltage. When the desired amplitude of the fundamental voltage is not very high, linear modulation may be used, in which the amplitude requirement can be met without introducing additional harmonics. When the desired amplitude of the fundamental voltage is high, however, additional harmonics have to be introduced. Existing methods of introducing harmonics to generate high-amplitude fundamental voltages are not optimized with respect to the number of switching operations, causing larger than necessary energy loss associated with the switching operations. Therefore, it is beneficial to reduce the number of switching instances while achieving the same high-amplitude fundamental voltage output.